1. Field of the Invention
The present invention relates to a laser welding method, materials and apparatus for welding materials coated with a film of a lo melting point substance by the application of a laser bear and the injection of an assist gas.
2. Description of the Background Art
In lap welding by means of a laser beam, a laser beam with sufficient energy density to weld materials is applied to material surfaces that are closely lapped without spacing. At the same time, assist gas is injected to enhance a cooling effect around the welding spot, to protect the laser beam lens from metal evaporation and from molten metal, and to protect the weld zone from ambient air, which tends to decrease welding quality. However, such a method is adequate for the welding of single materials which do not having film layers on their surfaces. Problems arise if the metal materials to be welded are those having films made of substances lower in melting point than their base material, e.g., galvanized steel plates, or if the metal material is welded in close contact, as described above,% with an unfilmed steel plate with the film layers locate inside. Specifically, the zinc which has a low melting point of 420.degree. C. will vaporize while the steel is in a molten state. This will cause the occurrence of an explosion during welding with the laser beam, the splashing of the weld zone, and the formation of a hole, especially on the laser application side, thereby resulting in an unfavorable welding quality.
To cope with this problem, one conventional solution is to provide a space between the joint surfaces of fused members to be lapped. For example, FIG. 7 shows an approach taught in Japanese Laid-Open Patent Publication No. 55478 of 1987 wherein a space is provided in a weld zone. Referring to FIG. 7 the numerals 1 and 2 indicate fused members, 1a, 1b and 2a, 2b represent galvanized layers of the fused members 1 and 2, respectively, 9 designates a space provide between the joint surfaces of the fused members 1 and 2, 6 denotes a laser beam, 7 indicates assist gas, and 4 indicates a fuse zone.
FIG. 9 is a schematic diagram of a laser beam head which applies a laser beam and injects assist gas to materials to be welded. In FIG. 9, 1 indicates materials to be welded (fused members; 6 a laser beam, 7 an assist gas; 12 a laser beam lens; 3 an adjustment mechanism for the laser beam lens 12; 5 a nozzle from which a laser beam 6 and assist gas 7 are applied and injected and 15 denotes an adjustment mechanism of a nozzle.
The operation of the conventional design shown in FIG. 7 will now be described. In FIG. 7, the fused members 1 and 2 are lapped and comprise galvanized steel plates of 0.4 mm thickness having the galvanized layers 1a, 1b and 2a, 2b on their front and back surfaces, respectively. A recess 10 of 50.mu.m depth is formed in part of the joint surface of the lower fused member 2 of the lapped members. By lapping the two members, the blocked up space 9 is formed by the recess 10. This space 9 is a portion where the two members are to be Welded, and has a wider area than the fuse zone of the two members resulting from welding. With the exception of this space 9, the two members are in total contact with each other.
The laser beam 6 is applied to the fused members 1, 2 (formed as described above) toward the space 9 from the fused member 1 side, whereby the fused members 1, 2 are welded. In this welding, he galvanized layer 1a on which the laser beam 6 is applied produces an explosive phenomenon, since it gasifies almost instantaneously and escapes to the outside. The galvanized layers 1b and 2a are opposed to each other with the space 9 in between. With this arrangement, an explosive phenomena similar to one produced by the galvanized layer 1a will be caused. However, most of the gasified layer material will expand without mixing into the fuze zone 4 that is generated by the application of the laser beam to the fused members 1, 2. Since the recess 10 has a wider area than the fuse zone 4, it acts as a contained volume into which the evaporated matter, comprising mainly the galvanized layers 1b, 2a, can enter. This containment will prevent a hole-making phenomenon from occurring during laser welding.
FIG. 8 shows a known welding method wherein a filler wire is inserted. In FIG. 8, 1 and 2 indicate fused members, 1a, 1b and 2a, 2b denote galvanized layers of the fused members 1 and 2, respectively, 6 represents a laser beam, 8 designates a filler wire, and 4 and 11 indicate fuse zones.
The operation of the method shown in FIG. 8 will now be described. Referring to FIG. 8, the fused members 1 and 2 to be welded are galvanized steel plates of 0.8 mm thickness having the galvanized layers 1a, 1b and 2a, 2b on their front and back surfaces, respectively, and are disposed in a "butted" form. The laser beam 6 is applied to the top surface. At the same time, the filler wire 8 is inserted into the top surface of the butted position, whereby the fused members 1, 2 are laser welded. The diameter and feedrate of the filler wire 8 will depend on the gap between the butted members, the thickness of the fused members and the kind of fused member that is utilized. As the materials are made thicker and/or the gap is made larger, a larger diameter wire is required, but the feedrate is smaller. Also, as the galvanized layers 1a, 1b and 2a, 2b become thicker, molten metal from the filler wire and the fused members settle into each other more poorly, causing the fuse zone 4 to be disturbed or splash. This will cause deposits 11 to stick to the welded member surfaces.
Since the laser welding method known in the art operates as described above, an explosive phenomenon caused by vaporized zinc can be prevented when there is welding of galvanized steel plates whose base materials have been filmed with zinc, a low melting point substance. However, this process is more expensive and time consuming since the work of recessing parts to be welded is added to the welding process. Furthermore, the recess must be detected and the beam application position located exactly prior to welding. In addition, a swell is made on the back side of the joint portion, thereby harming the appearance of a product. Further, since the amount of zinc that is gasified will increase in proportion to the increase in zinc used for galvanization the volume of the space 9 must be changed in response to that increase.
Furthermore, the welding method wherein the filler wire is inserted has disadvantages in that an uneven bead in the fuse zone reduces welding strength and that a postprocess for removing molten metal sticking to the surface is required.